My Nonna (grandmother) made this frittata in Italy as a child and teen, and they often ate it for brunch after church, slightly cooled, with toast spread with ricotta and fresh coffee grounds. You can add any variation of veggies and cheeses; that’s a big reason I love it!
Preheat oven to 350°F. Grease a pie plate or an 8x8-inch baking dish.
In a bowl, beat eggs with milk until combined. Set aside.
In a skillet, warm oil over medium heat. Cook onions and garlic for 2 to 3 minutes, or until soft. Add vegetables and cook, stirring, until softened. Season with salt, pepper, and fresh herbs (if using).
Slide the veggies into prepared pie plate, sprinkle cheese on top, and pour eggs evenly over all. Bake for 20 minutes; eggs should be set and firm, but not too dry. Let sit for 10 to 15 minutes before serving. Leftovers can be refrigerated and served cold.